In recent years, the increasingly widespread use of display device alternatives to the cathode ray tube (CRT) has driven the demand for large-area electronic arrays. In particular, amorphous silicon and laser recrystallized poly-silicon are used to drive liquid crystal displays commonly used in lap-top computers. However, fabricating such large-area arrays is expensive. A large part of the fabrication cost of the large-area arrays arises from the expensive photolithographic process used to pattern the array. In order to avoid such photolithographic processes, direct marking techniques have been considered an alternative to photolithography.
Examples of direct marking techniques used in place of photolithography include utilizing a xerographic process to deposit a toner that acts as an etch mask and using an ink-jet printhead to deposit a liquid mask. Both techniques have corresponding problems. Toner materials are hard to control and difficult to remove after deposition while controlling feature size of a printed liquid mask is difficult.
Small printed features have been obtained using ink-jet printheads. Special piezoelectric ink-jet printheads allow generation of low droplet volumes. However, even with the special printhead, the small size of features critical to the fabrication of large-area arrays has been difficult to achieve because of the surface tension between the droplet and the wetted surface. Typically, complete wetting is needed to form a good contact in order to prevent undercutting of the printed mask. However, as the surface is wetted, the liquid droplet tends to spread out making it difficult to control feature size. When used to pattern elements on a substrate, the droplet spreading results in undesirably large printed feature sizes. Decreasing the wetting properties of the substrate surface results in unreliable patterning due to poor adhesion of the droplet to the substrate. The poor adhesion can cause defects such as discontinuities in lines being fabricated. Variation in wetting properties across a substrate also presents problems as the droplet size must be adjusted to accommodate different materials to be patterned.
Thus an improved method for masking a substrate to be etched is needed.
The present invention relates generally to the field of device processing. In particular the invention relates to a method and apparatus for masking a surface by ejecting droplets of a phase-change masking material from a droplet source onto a surface to be modified or etched. In order to avoid extended wetting and still achieve improved adhesion between the masking material and the surface to be etched; the surface is maintained at a temperature such that the masking material remains in a liquid phase for only a brief period after contact with the surface.
In one embodiment of the invention, the temperature of the surface to be etched is maintained below the freezing point of the phase-change masking material such that after deposition, the masking material rapidly converts from a liquid to a solid. In a second embodiment, the temperature of the surface to be etched is maintained above the boiling point of a liquid that carries a suspended masking material such that the liquid carrier rapidly evaporates after contact with the surface to be etched. One particularly suitable embodiment of the invention utilizes a phase-change organic material that solidifies at room temperature making such phase-change organic materials suitable for room temperature operation of the described processes.
After deposition of the masking material, the surface is etched to remove material not masked by droplets of the phase-change masking material. After etching the surface masked by the phase-change masking material, the phase-change masking material is removed from the surface.